The invention relates to a method of determining the presence and location of seismic reflections from the earth's subsurface formations.
In marine seismic exploration, seismic energy is generated in the water layer above the earth's surface, is reflected from subsurface interfaces between layers of the earth, and is received by hydrophones located in the water layer or on the earth's surface at the bottom of the water layer. The hydrophone signals are recorded in the form of a seismic time section. This seismic time section contains information which can be used to represent the characteristics of the subsurface formations.
Primary reflection signals on the seismic time section indicate the presence of a subsurface reflecting interface, and time occurances of these primary reflection signals represent the depth of the subsurface reflecting interface. Furthermore, the time-shift or attitude of a primary reflection from trace-to-trace indicates the dip or slope of the subsurface interface.
It has long been known that seismic data collected over a deep and undulating water bottom and viewed as seismic time sections, represent a highly distorted picture of subsurface interfaces. Conventional data processing techniques of velocity analysis, normal moveout corrections, and CDP stacking is based on an undistorted model for which relatively flat underlying structures are characterized by hyperbolic moveout on seismic time sections. However, in the presence of significant water bottom distortion, flat, subsurface reflecting structures cannot be characterized by hyperbolic moveout. Consequently, stacking velocities not only become more difficult to estimate, but must be estimated at more frequent intervals in order to produce an acceptable stacked time section. Furthermore, the stacked section will be of less quality (i.e., display less trace-to-trace coherency at reflecting interfaces), and the interpretation of stacking velocities becomes more difficult.